On Tue, 4 May 2004, Alex Fraser wrote:
> ...it occurred to me that you surly could steer the rocket with the
> various methods, but you could not accurately position the vehicle. If
> you are trying to land on the X mark (or fly through a small fixed
> donut) and it is important to really hit that mark you will find you
> move the machine sideways by altering the thrust line (vanes, gimbels or
> what ever)...
Actually, most of the methods mentioned can be used to position a vehicle
in hover, which seems to be what you're really thinking of. RCS, fluid
injection, CG shift, all work for that. About the only ones which don't
are the aerodynamic ones, which require some amount of forward motion.
And you can do a precision landing even with aerodynamic controls, in
principle, by simply not slowing below minimum controllable speed until
the last instant. (Admittedly this may make for a hard landing.) Real
vertical landings -- as opposed to what you might see in an airshow --
are *not* done by slowing to a hover well off the ground and then slowly
descending. They're done as a rapid, decelerating descent, with descent
rate reaching zero roughly at touchdown.
Henry Spencer
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